The RFID systems enable data to be transmitted over a very short distance (from a few centimeters to a few meters, or, exceptionally, more) between a reader and a tag of small dimensions. The reader sends a signal at a radiofrequency which transports energy and information (for example, an interrogation) to the tag. The tag is activated by the energy received and it transmits to the reader information in response to the received information or interrogation. The information may be a simple numerical identification of the tag and therefore, indirectly, of the object to which the tag is affixed. The tag includes a radio antenna which makes it possible to receive the signal at the frequency sent by the reader and which makes it possible to send back a radio signal modulated by the information to be transmitted to the reader in return. The modulation is done in the tag which may include, for this purpose, an integrated circuit chip. The tags are sometimes provided with a battery to handle auxiliary functions, but more often than not, when their function is primarily an identification function, they are entirely powered by the radiofrequency energy received from the reader.
The modulation performed by the reader is more often than not an amplitude modulation of a carrier frequency, but efforts are now also being made to do a frequency or phase modulation, allowing for higher information bit rates. The modulation performed by the tag may be a load modulation, that is to say, a modulation of the impedance of the antenna, this modulation reacting on the impedance of the sending/receiving antenna of the reader. The interest here is mainly focused on the modulation done by the reader and on the demodulation done by the tag.
When the modulation by the reader is a frequency or phase modulation of a signal with a central carrier frequency F0, it is essential to be able to demodulate the information in the tag and, for this, it is essential to have a signal at the same central frequency F0 in the tag. In the usual radiocommunication systems operating in frequency or phase modulation mode, the receiver generally has a local frequency generator aligned on the central frequency of the radiofrequency signal that is to be demodulated. The local generator produces the demodulation by frequency transposition to an intermediate frequency or to the baseband. The demodulators can also use a phase-locked loop receiving the signal from a local voltage-controlled oscillator (VCO) and aligning the frequency of this signal on the received frequency. With modulations of a very particular type, it is sometimes possible to dispense with a local oscillator, but this is not possible in the general case.
Now, in an RFID tag, there is not a lot of space for this frequency generator or this local oscillator and there is not a lot of energy available to operate it. A controlled-frequency local oscillator or a standalone frequency generator based on an integrated circuit chip generally require components external to the chip and it is not desirable to have such external components in the tag, for bulk reasons.